Several different measures have been proposed for straightening up the leakage flux, for example by locating, adjacent to the windings, bodies of high permeability, wirewound coils traversed by the winding current, or shields of electrically conducting material (see, e.g. U.S. Pat. Nos. 3,142,029 and 4,012,706). However, these measures only result in a limited reduction of the edge current density.